Such lighting systems are supplied from an onboard energy source which is typically the battery of the vehicle. Vehicle batteries used at the present time generally deliver a nominal voltage of around 12 volts. However, it is normal practice to find that there are over-voltages at the terminals of the items of equipment supplied by the battery. These over-voltages may be of short duration, and may not perturb the operation of the apparatus except over a time interval short enough to be acceptable.
However, over-voltages in the power supply network may often be prolonged, especially as a result of the discharge of capacitive elements, or because an alternator regulator is defective. In the absence of any additional regulation of voltage directly at the terminals of the apparatus, the disturbance to the operation of the latter will therefore persist.
In the case of headlights, such a prolonged over-voltage may typically reach 18 volts and can cause the light intensity emitted to be increased. This can infringe photometric regulations and cause dazzling, which is tiresome or even dangerous to drivers coming in the opposite direction.
The over-voltages can also be voluntary, because manufacturers sometimes increase the power distributed in the vehicle in order generally to compensate for the various voltage losses that are linked to the increasing number of items of electrical equipment mounted in a vehicle, such as electrical power assisted steering, electrically governed suspension systems, and so on. In this context, headlights are such that, under these circumstances, they will emit light of too high an intensity, which, apart from the dazzling effects already mentioned above, also involve a considerable shortening of the useful life of filament lamps in the headlights.
In addition, in order to solve the problem of the increase already mentioned above, in the power consumed by the various onboard electrical items of equipment, it has been arranged that the nominal supply voltage delivered by the battery of the vehicle should be substantially increased. Thus, some vehicles have batteries of 24 or 42 volts for example.
The use of existing apparatuses which do not have intermediate adjusting means for the voltage is then excluded, because the voltages delivered at the terminals of such apparatus is too different from the nominal voltage of each respective apparatus, which is of the order of 12 volts.
In the case of filament lamps, which are the type most often used in headlights, it is certainly possible to envisage that the lamps should have a supply voltage corresponding to the battery voltage of the vehicle, of 24 or 42 volts for example. However, the constraints imposed on design and manufacture of the filament (which has to have a very fine double coil so that it can be brought to incandescence, but which must also be quite strong so that it will not be damaged by the supply voltage) has made such lamps expensive and fragile. In addition, the filament of such a lamp is not well adapted to the photometric requirements imposed by certain so-called "free surface" reflectors.
Another solution would consist in the use of a second voltage source which is dedicated to the lighting system, or the use of a specific electrical network for lighting purposes, connected to the battery of the vehicle through a voltage reducer. However, this would involve major complexity and extended length in the wiring network of the vehicle, which is something manufacturers try to avoid, given the high costs involved in wiring.
One solution that can resolve the above mentioned drawbacks linked to persistent or permanent over-voltages or to an increase in the nominal voltages delivered by the vehicle battery, consists in associating with each lamp of a headlight or indicating signal display a voltage regulating apparatus which firstly reduces the voltage to the rated or nominal value of the headlight or indicating light in the case of general supply of the vehicle with a voltage greater than the rated voltage, and secondly, regulates the voltage so as to minimise fluctuations in the voltages that may occur across the lamp.
One such voltage regulating device has been described by the Company Valeo Vision in French patent specification No. FR 2 707 021. In the arrangement described in that document, a n electronic circuit modules the vehicle battery voltage and a periodic voltage with square wave pulses for supplying the lamps of the illumination equipment. The periods of the pulses of the supply voltage are so defined that there mean value is adjusted on a given rated value corresponding to the nominal power supply voltage of the lamps.